prog1 (ANSI and GNU Common Lisp Document)

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#### 5.3.58 prog1, prog2 [Macro]

`prog` 1 ⇒ first-form {form}* result-1 `prog` 2 ⇒ first-form second-form {form}* result-2

#### Arguments and Values::

first-form—a form; evaluated as described below.

second-form—a form; evaluated as described below.

forms—an implicit progn; evaluated as described below.

result-1—the primary value resulting from the evaluation of first-form.

result-2—the primary value resulting from the evaluation of second-form.

#### Description::

prog1 evaluates first-form and then forms, yielding as its only value the primary value yielded by first-form.

prog2 evaluates first-form, then second-form, and then forms, yielding as its only value the primary value yielded by first-form.

#### Examples::

     (setq temp 1) ⇒  1
     (prog1 temp (print temp) (incf temp) (print temp))
     |>  1
     |>  2
    ⇒  1
     (prog1 temp (setq temp nil)) ⇒  2
     temp ⇒  NIL
     (prog1 (values 1 2 3) 4) ⇒  1
     (setq temp (list 'a 'b 'c))
     (prog1 (car temp) (setf (car temp) 'alpha)) ⇒  A
     temp ⇒  (ALPHA B C)
     (flet ((swap-symbol-values (x y)
              (setf (symbol-value x)
                    (prog1 (symbol-value y)
                           (setf (symbol-value y) (symbol-value x))))))
       (let ((*foo* 1) (*bar* 2))
         (declare (special *foo* *bar*))
         (swap-symbol-values '*foo* '*bar*)
         (values *foo* *bar*)))
    ⇒  2, 1
     (setq temp 1) ⇒  1
     (prog2 (incf temp) (incf temp) (incf temp)) ⇒  3
     temp ⇒  4
     (prog2 1 (values 2 3 4) 5) ⇒  2

#### See Also::

multiple-value-prog1 , progn

#### Notes::

prog1 and prog2 are typically used to evaluate one or more forms with side effects and return a value that must be computed before some or all of the side effects happen.

     (prog1 {form}*) ≡ (values (multiple-value-prog1 {form}*))
     (prog2 form1 {form}*) ≡ (let () form1 (prog1 {form}*))

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